The insertion of a security thread into security document paper such as that used for printing bank notes is known. Such threads are typically in the form of an elongate strip typically formed from a plastics film, one surface of which is itself coated with a thin metallic film, and sandwiched between the metal film and the plastics film are regions of magnetic material which are spaced apart and are of such a length (measured in the length direction of the thread), that when moved relative to a suitable electromagnetic reading head, can induce electrical signals having characteristics which can be de-coded to determine the pattern of magnetic material in the security thread.
It has also been proposed to incorporate symbols, letters, numerals or other visually decipherable or machine readable data along the length of the thread and to arrange that this visually and/or machine readable material can be identified at least on one side of the thread and can be inspected (visually or by machine reading) when the thread is incorporated in a security document or bank note.
In particular the readable data may comprise a short description of the document within which the thread is incorporated, and in the case of a bank note may be the currency and denomination of the note.
In U.S. Pat. No. 5,516,153 there is described a security thread in which a plastic carrier is coated with a metal layer to form recessed characters on which is overprinted bars of magnetic ink. Alternatively a thin layer of magnetic pigment may be applied all over the metal layer. Reference is also made therein to EP-A-0330733 which discloses a security thread having a substrate carrier layer made of a tear-resistant material such as polyester.
Security threads are also described in EP Specifications 0400902, 0426801, 0498186, 0659587 and 0613786.
It is an object of the present invention to supply a method and apparatus for producing a preferred form of a composite security thread ie. one having a metallic film for electrical continuity, magnetic encoding to allow the thread to be read using a magnetic reader, and bearing visible data or markings which can be seen by the naked eye.